2.1. INSECT VIRUSES AND OCCLUSION BODIES
Baculoviruses are a group of viruses which are pathogenic for insects and some crustaceans. The virions of these viruses contain rod-shaped nucleocapsids enclosed by a lipoprotein membrane. Two morphologically distinct forms of baculovirus are produced by infected cells: the nonoccluded virus and the occluded virus. The nonoccluded virus is synthesized early after infection; nucleocapsids are assembled in the nucleus and acquire an envelope by budding through the plasma membrane to become extracellular virus. In occluded baculoviruses, the virions are embedded in the nucleus in large protein crystals, termed polyhedra or occlusion bodies.
Baculoviruses are members of the family Baculoviridae and the genus Baculovirus. This genus is composed of three subgroups of viruses: the nuclear polyhedrosis viruses (NPV), the granulosis viruses, and the non-occluded viruses.
NPV have occlusion bodies which are polyhedral to cuboidal in shape, and 1-15 um in diameter. The lipoprotein membranes contain either single nucleocapsids (SNPV) or multiple (up to 39) nucleocapsids (MNPV) per envelope. Up to 100 virions can be embedded in a single occlusion body (Vlak, J. M. and Rohrmann, G. F., 1985, The Nature of Polyhedrin. In Viral Insecticides for Biological Control. Academic Press, pp. 489-542) Examples of this group of viruses include Autographa californica NPV (AcNPV), Heliothis zea NPV (HzNPV), and Bombyx mori NPV (BmNPV). Comparison of DNA sequences of total viral genomes reveals a less than 2% homology between HzSNPV and AcMNPV, whereas a comparison among various MNPVs shows a greater degree of homology (Smith, G. E. and Summers, M. D., 1982, Virol. 123:393-406). HzSNPV is currently produced and sold in the United States for use as an insecticide under the trade name Elcar.TM..
The granulosis viruses have round to ellipsoidal occlusion bodies which are 0.1-1 um in size. Each occlusion body contains one singly-enveloped nucleocapsid (Vlak, J. M. and G. F. Rohrmann, supra).
Baculoviruses contain double-stranded, circular DNA molecules, which range from 60-110.times.10.sup.6 daltons. The prototype of the Baculoviridae family is AcNPV, which has a genome of approximately 82-88.times.10.sup.6 daltons (Miller, L. K., 1981, A Virus Vector for Genetic Engineering in Invertebrates. In Genetic Engineering in the Plant Sciences. Praeger Publishers, New York, pp. 203-224). AcNPV replicates in the nucleus of infected insect cells. Two forms of virus are produced as a result of wild-type AcNPV infection, occluded and non-occluded virions.
The apparent role of the occlusion body in the virus life cycle is to provide stability outside the host insect by protecting the virus from inactivating environmental factors. Ingested occlusion bodies dissolve in the alkaline environment of the midgut, releasing virus particles for another round of infection. The occlusion body consists predominantly of a single, approximately 29,000 dalton molecular weight polypeptide, known as polyhedrin (Vlak, J. M. and Rohrmann, G. F., supra; Miller, L. K, supra). This protein forms the paracrystalline lattice around the virions, and is present as a multimer. Polyhedrin is produced in enormous amounts during the course of viral infection, late after viral replication. As there is no evidence of gene amplification (Tjia, S. T., et al., 1979, Virology 99: 399-409), it is probable that the polyhedrin promoter is an extremely efficient one.
Plaque-purified isolates of Heliothis zea SNPV have been characterized, and one such isolate has been cloned (Corsaro, B. G. and Fraser, M. J., 1985, Society for invertebrate pathology, XVIII Annual Meeting, August 4-8, 1985, Ontario, Canada, Abstract 75).